The present invention relates to an operation system for a furnace such as a blast furnace, a shaft furnace, or the like.
A blast furnace is a gigantic, hermetically sealed, high-temperature reaction furnace covered with a thick layer of a refractory substance. It is quite difficult to grasp the behavior of the charges and the gases in the furnace in a correct and accurate manner. However, for the purpose of effecting the improvement of the productivity of the furnace and the quality of pig iron, it is imperative that the conditions in the furnace (hereinafter referred to as the furnace conditions) be maintained and controlled in a proper and stabilized manner. Here, the conditions in the furnace, generally expressed by the terms of the furnace conditions, will be classified into the shape of more concrete phenomena, and the mutual relations among them, also the positions thereof in the operation of a blast furnace will be described in a comprehensive manner.
Generally, the furnace conditions are roughly classified into air-permeable conditions in the furnace and furnace thermal states. The former, or the air-permeable conditions, are further subdivided into the following categories.
(i) Air-permeability in a narrow sense, which is grasped in terms of the gas pressure loss in the furnace or the fluctuations in the blast pressure; PA1 (ii) Conditions of the downward movement of the charges, which is grasped in terms of the state of taking shape of such defective downward movement of the charges as hanging, slip, or the like; and PA1 (iii) The gas flow distribution in the direction of the radius of the furnace, distribution of the velocity of the downward movement of the charges, and distribution of the thickness of the layer.
The latter, or the furnace thermal states, are further subdivided into distribution of the temperature in the direction of the height of the furnace and distribution of the temperature in the direction of the diameter of the furnace.
Especially, the temperature in the directly reducing zone at the lower portion of the furnace (that is to say, the level of the furnace thermal state, the pig iron melting temperature); the level of the concentration of Si contained in pig iron and the state of the fluctuations therein; the temperature of the body of the furnace, including the shaft section and the like; and the state of the temperature of the gas at the top of the furnace, constitute the key items of the furnace thermal state among others. Furthermore, the composition of the gas at the top of the furnace, including, for instance, CO, CO.sub.2, H.sub.2 and N.sub.2, whereupon the conditions of reduction of ore at the shaft section and the lower portion of the furnace are calculated, is also included as one of the key items of the furnace thermal state, since the said conditions constitute such items which influence the level of, especially, the endotherm attending upon the directly reducing reaction.
Next, described below will be as to the conditions of the permeability and the thermal state, especially the interrelation between these and the role thereof to be occupied in the operation of a blast furnace.
First, the conditions of the permeability are virtually determined by the physical properties of the charges to be charged into the blast furnace and the conditions of filling in the blast furnace. For instance, in case the particle size distribution of the coke, the ore, and the like constituting the charges (especially, the ratio of mixing of finely divided particles or finely divided powder) should extend (spread out), or the strength thereof should decrease, the permeability in the furnace is deteriorated. Therefore, to cope with such a situation, a countermeasure is employed of such a category that sieving is emphatically conducted prior to charging, to thus effectuate proper prevention of finely divided powder from being mixed into the furnace as much as practicable. Furthermore, control of the strength of coke and ore is carried out in a rigid manner, whereby such deterioration in permeability as is attributable to the physical properties of the charges has recently come to be reduced. Rather, the behavior of filling the charges into the furnace has come to carry considerable weight for the operation of a blast furnace. To put it otherwise, the conditions of the distribution of the thickness of the layers of coke and ore, the pattern of the shapes thereof (which is otherwise termed distribution of charges), and the conditions of the distribution of the velocity of descent, in the direction of the radius of the filling layers in the furnace, have come to be learned to the effect of exercising a considerable influence over the conditions of permeability and the furnace thermal state. Because coke is larger than ore in terms of the means particle diameter, their results in the reducing a great deal the resistance to permeation. Furthermore, while ore is subjected to softening and fusing in a high-temperature range of 1,000.degree. C. or over, to thus form a fused layer featuring a high level of resistance to permeation, coke, or the part thereof, maintains a virtually solid state in the furnace, except such a case wherein coke is subjected to combustion and extinction in the combustion zone arranged before the tuyere of the furnace. For this reason, the permeability in the furnace may well be considered as to be determined by the conditions of filling the furnace with coke (to put it otherwise, distribution of the thickness of the layer of coke in the radial direction of the filling layers in the furnace or in the direction of the height of the furnace).
Now, in the case of the operation of a blast furnace, it is a conventional practice that coke and ore are charged into the shape of lamellar layers. Distribution of the thickness of the coke layer and distribution of the thickness of the ore layer have a close and inseparable relation with each other. Therefore, it is quite important to grasp the state of the both in the filling layers thereof, and to effectuate the control thereof in a proper manner. Furthermore, for the purpose of ensuring favorable permeability, it is recommendable that the thickness of the coke layer in the vicinity of the center of the furnace be increased, and that the thickness of the coke layer to be charged each time to be increased, to thus enable the gas to run through the coke layer readily enough.
However, in case the gas is so caused as to run through the coke layer in the furnace up to an excessive level, with too much emphasis placed on the permeability, to the contrary, the reaction of contact of ore with CO and H.sub.2 in the indirectly reducing zone area in the top portion of the furnace is subjected to reduction. As a result thereof, the reduction efficiency of ore is decreased, until the direct reduction constituting a remarkable endothermic reaction is increased, thus resulting in a shortage in furnace heating. In such a case as this, now that the thickness of the ore layer in the vicinity of the furnace wall or in the intermittent portion in the radial direction is increased, such a situation results in decreasing the calorific value required for fusing to be effectuated at the time a thick layer of ore comes descending down to a position immediately above the combustion zone of the tuyere, hence an increase in the fusing load, until such might possibly lead to the deterioration in the conditions of the furnace by cooling or the like.
Besides, in case the thickness of the ore layer in the vicinity of the furnace wall is decreased, and the thickness of the coke layer in the said vicinity is increased, the fusing load in the combustion zone at the tuyere is alleviated. However, in this case, the flow of the gases is increased in the vicinity of the furnace wall, to the contrary. For this reason, an increase in the damage of the body of the furnace by gas attack, or an increase in frequency of coming-off of the adherend to the furnace wall, is thus caused to entail. Such might possibly constitute a cause of increasing the fluctuations in furnace heating in some case.
As set forth in the preceding paragraphs, the behavior of filling coke and ore in the filling layers in the furnace (especially, distribution of the thickness of the layers of the charges in the direction of the diameter of the furnace or in the direction of the height of the furnace) exercises a profound influence over the conditions of permeability and the conditions of furnace heating of a blast furnace (to put it otherwise, the conditions of a blast furnace). Therefore, the said items constitute important items of the object of control in the execution of the operation of a blast furnace.
However, now that a blast furnace is such a gigantic high-temperature reaction furnace as is covered with a thick layer of proper refractory, it is quite difficult to effectuate infallible detection of the behavior of the charges in the filling layer in the furnace. Such a means as proves effective enough for detecting in a direct manner the behavior of the charges in the direction of the diameter of the furnace or in the direction of the height of the furnace has thus far remained to be developed. For this reason, the only method employed for this purpose has been such an indirect one as simply serves for drawing an analogical inference. For instance, with regard to the distribution of the charges, such an attempt as is specifically designed for the purpose of detecting the distribution and the shape of the surface of the charges at the top of the furnace, wherein the head of a chain or a wire is caused to descend to the surface of the charges immediately following the charging thereof, to thus measure the depth of charging from the standard line, and the distribution of the thickness of the layers of the charges is to be detected, by taking the balance between the measured value thus obtained and the measured value obtained likewise at a time subsequent to charging as a criterion thereof. To add up thereto, such a method as is specifically contrived for measuring the distribution and the shape of the surface of the charges from the top of the furnace, by making use of a microwave, has been also introduced. However, it has been thus far confirmed through a series of model experiments, that the distribution and the shape of the charges immediately following the charging are subjected to fluctuations in the filling layers up to a fairly high level, due mainly to the disparity in the property values of the ore and the coke to be charged into the furnace from the top thereof and that in the momentum of the ore and the coke at the time of charging thereof.
The reason why the distribution and the shape at the time of charging and the distribution and the shape in the filling layers are in disparity in such a manner as is set forth above is assumed to rest with the undermentioned factors. To put it in concrete terms, now that the angle of repose of coke is larger than that of ore, the distribution and the shape immediately following the charging are rather larger in terms of the angle of inclination at the time of the charging of coke than at the time of the charging of ore. However, in the case of the charging of coke, followed by the charging of ore at the subsequent stage, the momentum of the ore at the time of the charging thereof is larger than that of the coke by as much as three to four times. For this reason, the impact force thereof so functions as to push the layer of the coke charged immediately prior thereto in the direction of the center of the furnace or in the direction of the furnace wall. As a result thereof, the distribution and the shape of the layer of the coke, as a whole, assumes a flat shape. To put it otherwise, the angle of inclination of the coke in the furnace becomes smaller than that of the ore. Thus, the distribution and the shape of the coke immediately charged at the time of the charging of the ore becomes fairly disparate from the distribution and the shape of the coke at the time of the charging thereof. Furthermore, pointed out as another factor of reducing the angle of inclination of the coke is such that the bulk specific gravity of coke is so light as to be approximately 0.5, while the bulk specific gravity of ore is approximately 2, wherefrom coke is prone to be pushed upward by virtue of such lifting power as is given birth by gases rising upward from below in the course of the operation. In view of the above-mentioned reasons, it must be considered that the actual distribution of the thickness of the layer in the charge filling layer in the furnace has thus been already subjected to fluctuations up to a fairly high level, even in case the shape of the surface of the charges before and after the charging of coke and that of the charges before and after the charing of ore are measured, respectively, and the distribution of the thickness of the layers, including the layer of the coke and the layer of the ore, in the radial direction is found on the basis of the balance of the charging depth between the both.
There has also been introduced a method wherein a magnetometer is fitted in place in the vicinity of the furnace wall or the internal furnace wall, to thus detect the behavior of the charges present in the vicinity of the furnace wall. However, such a method as this one is incapable of detecting but the behavior of the charges within the range of approximately a few score centimeters at best in terms of the distance apart from the internal wall of the furnace. For this reason, it cannot but say that the detection of the behavior of the charges in the direction of the diameter of the furnace within such a blast furnace whereof the inner diameter is 10 m or even more, like a gigantic blast furnace of the latest design, is far beyond practicability at all.
By the way, besides the above-mentioned attempt of directly detecting the behavior of the charges, there has also been introduced a method wherein a horizontal sonde is inserted in place or laid over in the direction of the diameter of the furnace at the top portion of the furnace, to thus conduct measurement of the distribution of the temperature or the distribution of the composition of gases. Furthermore, introduced is such a method wherein the pattern of the temperature on the surface of the charges is measured from the top of the furnace by the employment of an infrared ray camera, to thus assume the distribution of the flow of gases or the distribution of the charges in the furnace in an indirect manner. However, this category of method of detecting the temperature and the composition of gases is what is serviceable only for grasping the distribution of the flow of gases and the distribution of the charges, specifically the trend thereof, simply qualitatively to some extent. In some case, the method of this category possibly involves a danger of providing even erroneous information. For instance, in the usual practice, when the temperature of the gases at some measuring point is high, or when the CO-to-CO.sub.2 ratio in the gases is beyond a reasonable level, the layer of coke is judged to be thick, while the layer of ore is relatively judged to be thin, in the internal region of the filling layer below the said measuring point; therefore, it is duly judged to the effect that the flow velocity of the gases in the said region is high enough, and that permeability is maintained in a favorable state. However, when the temperature in the said region is low, even in case the flow velocity of the gases is high enough, the temperature of the gases present at the top of the furnace is so detected as to be rather lower than the actual level. Furthermore, even in case the thickness of the layer of ore is relatively large, the CO-to-CO.sub.2 ratio is prone to be so detected as to be beyond a reasonable level, when the velocity of downward movement of the charges in the said region is slow, or when the temperature is so low that the indirect reducing reaction of ore by the CO gas is checked from taking shape in a proper manner.